1. Field of the Invention
This invention relates to a traction control system for a vehicle, and more particularly to a traction control system for a vehicle in which the engine output is feedback-controlled so that slip of the driving wheels of the vehicle converges on a predetermined target value.
2. Description of the Prior Art
There has been known a traction control system for a vehicle which controls the traction of the vehicle in order to prevent deterioration of the accelerating performance due to slip of the driving wheels caused by an excessive driving torque, for instance, during acceleration. In the traction control, when the rate of slip of the driving wheels becomes excessive due to an excessive driving torque, the engine output is suppressed and/or braking force is applied to the driving wheels so that the rate of slip of the driving wheels calculated on the basis of the rotational speed of the driving wheels converges on a predetermined target value.
When the vehicle is provided with an engine having a supercharger, the supercharging pressure generated by the supercharger is sometimes controlled as a part of the engine output control.
In the traction control where the engine output is feedback-controlled to converge the rate of slip of the driving wheels to the target value, the following problem sometimes arises.
For example, in some supercharged engines, the supercharging pressure generated by the supercharger is feedback-controlled in order to maximize the output performance of the engine while ensuring the durability of the engine. That is, for instance, a pressure sensor is provided downstream of the supercharger and the supercharging pressure is controlled so that the difference between the actual supercharging pressure detected by the pressure sensor and a target supercharging pressure set according to the operating condition of the engine is nullified.
On the other hand, when the traction control is effected by feedback control of the supercharging pressure, the supercharging pressure is controlled so that the difference between the actual rate of slip of the driving wheels calculated on the basis of the rotational speed of the driving wheels and a target rate of slip set on the basis of, for instance, the vehicle speed is nullified. When the former is larger than the latter, the supercharging pressure is reduced and when the former is smaller than the latter, the supercharging pressure is increased. When the vehicle goes into a high-.mu. road (with a surface having a high friction coefficient) having a large gripping force from a low-.mu. road (with a surface having a low friction coefficient) having a small gripping force, the rotational speed of the driving wheels lowers and the rate of slip reduces, and accordingly, the supercharging pressure is increased. However, since the supercharging pressure is not increased beyond the target supercharging pressure, the boost signal is kept output so as to converge the rate of slip on the target rate of slip if the rate of slip does not reach the target rate of slip by the time the supercharging pressure reaches the target supercharging pressure, whereby the traction control cannot be terminated despite the fact that the driving wheels are not slipping.
The similar problem arises wide in traction control systems where the engine output is feedback-controlled so that the actual rate of slip of the driving wheels converges on a target value.
In Japanese Unexamined Patent Publication No. 2(1990)-140437, there is disclosed a technical concept in which it is determined that the traction control is to be terminated when the throttle opening (the opening of the throttle valve of the engine) comes to conform to the accelerator opening (the amount of depression of the accelerator) which is controlled by the driver.
However, the prior art is directed to preventing the driving wheels from beginning slipping again when the throttle valve is opened after termination of the traction control and the engine output is increased, and is not based on the case where the traction control cannot be terminated due to feedback control of the engine output. At the same time, there is a possibility that the traction control is terminated when the throttle opening just temporarily comes to conform to the accelerator opening as well as a possibility that the driving wheels begins to slip again when the condition of the road surface abruptly changes, e.g., when the vehicle goes into a low-.mu. road gain, immediately after termination of the traction control. This deteriorates the reliability of the traction control system.
Depending on the running condition of the vehicle, the accelerator pedal is sometimes released during acceleration. When the traction control is being effected in such a case, the traction control variable is gradually reduced since the rate of slip of the driving wheels is reduced in response to reduction of the driving force due to release of the accelerator pedal. By the way, an indicator lamp is generally turned on while the traction control is being effected and turned off in response to release of the accelerator pedal. Accordingly when the accelerator pedal is depressed again after once released, the indicator lamp is turned on again unless the traction control variable has been converged on a normal state. Though giving rise to no problem so long as the driving wheels are slipping, this gives the driver a sense of incompatibility.
An attempt to overcome this problem by canceling the traction control variable upon release of the accelerator pedal will result in deterioration in response when the accelerator pedal is depressed again immediately after release of the same.